Rolling mills for hot-rolling metal are well known in the art. Examples are shown in U.S. Pat. Nos. 3,257,835, 3,317,994, 3,296,682, 3,517,537, 3,672,199, 3,766,763, 3,881,336, 3,881,337, 4,087,898, 4,106,319, 4,159,633 and 4,193,823, the teachings of which are herein incorporated by reference. Such rolling mills normally roll metal stock, such as bar or rod, between pairs of smooth finished work rolls, or bearings, in roll stands or reduce the gauge of steel prior to processing into other types of ferrous substrates.
Industries dependent upon robing mills as a step in a manufacturing process, as e.g., the steel and aluminum industries, have been constantly searching for means to reduce the wear on the rolls. Reduced wear on the rolls results in increased roll life, thus fewer roll changes are required which, in turn, results in an increase in mill production, measurable in terms of the number of units manufactured between roll bearing changes. In additions, savings are realized from fewer roll redressings and reduced roll inventor requirements.
The main source of roll wear is friction. As metal stock enters a series of work rolls in a rolling mill, the final gauge or thickness of the rolled work material is dependent upon several factors. These will include the type of work material itself, surface characteristic of the work material, temperature of the work material, line speed of the work material, the number and configuration of work rolls, size of the rolls and cooling capabilities of the like. As metal stock enters a deforming and/or shaping region, the stock is progressively bent by a series of rollers until it assumes a desired shape such as a tube of circular cross section. It will be readily apparent to one of skill in the relevant art that other rolled steel products may be produced by rolling mills, including rectangular cross-section tube, "C" and "U" shaped steel channels, and other complex cross-sectional shapes.
When surfaces of work rolls and metal stock are placed in contact, they do not usually touch over the whole of their apparent area of contact. In general, they are supported by surface irregularities which are present even on the most carefully prepared surfaces. Even small loads produce plastic flow of the irregularities at these regions of contact, and the asperities crush down until they are large enough to support the load. Metallic junctions are often temporarily formed at the regions of real contact by a process of welding, and these junctions formed between the mill rolls and stock are subsequently sheared by the relative motion of rolling. The immediate consequence of this welding and shearing action, as it applied to hot rolling of stock, is that work roll surfaces are worn by the progressive removal of work roll surface material, stock quality is diminished as ferrous material becomes imbedded beneath the work surface, the working life of work rolls is reduced as stock materials becomes adherent to the surface of the rolls, and geometry of the rolling pass is distorted.
Lubrication of the surface of the rolls has been found to be most effective for resisting or minimizing the effects of the abrasive processes generally described above. Typically, the deforming and/or shaping rolls are continuously sprayed with a coolant/lubricant which serves to lubricate the rolls, and to remove at least some of the surface deposits formed during the milling process.
Many benefits from the use of hot mill rolling lubricants accrue, in addition to increased production at lower costs, such as minimizing metal pickup from the workpiece and peeling or removal of metal from the roll bearings to the workpiece as it goes through the various stands of the rolling operation. Proper lubrication during the hot rolling metal also gives an improved product surface quality due to the improved surface condition of the work rolls.
The introduction of suitable lubricants to effect a separation of the contacting surfaces between rolls and work material is important to reduce the effects of welding-shearing of loaded and load carrying surfaces in terms of usable roll life and work material quality. Lubricant suppliers have attempted to capitalize on the abilities of certain organic materials to become inherently attached to the surfaces of the rolls by chemical actions of polar activity. Typical materials of this type are natural oils such as palm and rapeseed oils. Although possessing some value as lubricants, these oils are typically present as a carrier base. These natural oils and their synthetic counterparts are expensive, and their lubricity performance will usually deteriorate with the increased temperatures typical of hot rolling. More inexpensive lubricants for hot rolling applications are based on nonpolar petroleum mineral oils in conjunction with emulsifiers, which together provide only minimal lubrication because their synthetically induced wetting and attraction to the conventional roll surfaces is soon lost due to contamination and the effects of high temperature exposure. The arrangement and position of lubricant sprays relating to the roll surfaces is not always remedial in compensating for the inability of such lubricants to be attracted to and carried on the surface of the rolls.
In rolling mill lubrication processes, the amount of lubricant which is processed, i.e., misted, is referred to as "throughput." Throughput is expressed as a unit of weight or volume per unit of time, e.g., grams/hour, and is further broken down into the following three components: (a) dropout, or breakout, (b) reclassified oil, and (c) stray mist. Dropout is the amount of mist which is condensed in the lines and never reaches the reclassifier. Mist which is condensed in the distribution lines may be returned to the mist generator and remisted. Reclassified oil is the actual amount of lubricant which is applied to the surface being lubricated. Mist which is not applied to the surface being lubricated but rather escapes into the atmosphere is referred to as stray mist or stray fog. Since throughput is equal to (a)+(b)+(c), stray mist is obtained by determining the difference between the throughput and the sum of (a) and (b). Dropout, reclassified oil, and stray mist are often reported as a percent of throughput or can be represented as a ratio.
Many types of lubricants have been developed for lubricating the surfaces of the work rolls in a hot rolling mill to reduce roll wear due to friction. The lubricants can be combined with water to form a coolant-lubricant system which cools the hot work material while lubricating the surfaces of the work rolls. These lubricants are conveniently divided into two major groups: (1) those which form heterogeneous aqueous mixtures, i.e., more than one phase; and (2) those which form homogeneous aqueous solutions or apparent solutions, i.e., one phase.
Lubricants of group (1) are normally thought to have relatively low lubricity and relatively low wetting ability. They also are nonpolar and thus must be synthetically suspended in water (which is polar) by emulsifying or dispersing agents. Group (1) lubricants are therefore normally referred to in the art as oil-in-water emulsion lubricants.
Oil-in-water emulsion lubricants form a suspension of lubricant material in water, are often milky white in color, and are opaque. The lubricant base is normally refined mineral oil to which are added an emulsifier agent and detergent, so that the lubricant will form tiny, suspended droplets of various diameters when mixed with or added to water. Since emulsion lubricants are the least expensive, conventional oil-in-water emulsion systems have long been attractive from a cost standpoint and are generally preferred in high volume, high make-up systems. When used for cooling lubrication in mild to medium duty applications, oil-in-water lubricants are usually found to be an acceptable choice. In extreme pressure, or high temperature service such as hot rolling, however, satisfactory lubrication and extended roll life are compromised because the typical oil-in-water lubricant is subject to failure.
As previously mentioned, this type lubricant mixture is comprised of minute droplets of non-uniform size and held in water suspension by the action of emulsifier or dispersing agents. The ability to lubricate metal surfaces by the usual means thereby becomes dependent on sufficient numbers of these lubricant droplets transferring from the water carrier medium and attaching themselves to all parts to be lubricated or, more specifically, the smooth finished work roll surfaces. Furthermore, it has been established that this ability to "plate out" or wet smooth finished metal surfaces is not shared by all lubricant droplets but is characteristic of only a few whose physical size fall within a relatively narrow range of diameters. In general, of the total lubricant content expressed as per cent volume of the working emulsion, only a very small amount is actually beneficial in reducing roll wear. High temperature, dissolved metal ions, hard water ions, gear box lube contamination, mechanical shear forces, and improper pH control are all forces which act to segregate the size of droplets to levels outside the range which is known to be useful. Considering the above description of lubricant dispersion in water, the mechanics of lubricant transfer to metal surfaces, and the comparatively low lubricant potential available even under conditions thought to be ideal in the prior art, oil-in-water emulsion systems have been considered by the industry to be inadequate in providing lubrication and roll life improvement in the more demanding applications. A better alternative was thought to be found in the more expensive water miscible rolling lubricant of group (2). Alternatively, compositions assuming the properties of stable dispersions, as opposed to emulsions, offer alternatives to the more expensive water-miscible compositions.
Roll life improvements over conventional oil-in-water systems are recognized by the industry with the use of miscible lubricant systems which can at least partially justify the increase in lubrication cost. However, polar lubricants also have limited usefulness in high temperature applications because the polarity induced boundary film is destroyed by extreme heat. Despite the increased expense of operation, the industry trend has been toward the use of rolling lubricants that form miscible solutions or mixture in water which are thought to be normally better able to perform the vital role of lubrication because the industry has assumed that they are less subject to influences which inhibit lubrication of metal surfaces than oil-in-water emulsions.
Despite the industry trend toward the use of water-miscible compositions of type (2), there remains a need in the field for lubricant compositions which can provide the enhanced lubricity of the type (2) compositions, while at the same time providing the cost savings of oil based lubricants, and the essential ability to withstand the high temperature associated with more demanding milling operations. In addition, an ideal lubricant composition must also exhibit good lubricity, oxidation stability, antiwear and extreme pressure properties, antirust/anticorrosion properties, and possibly other characteristics dependent upon the particular application involved. The lubricant must also be essentially free from undesirable waxes. Waxes can build up in the reclassifier heads and cause restriction or complete blockage thereof. In either event, insufficient lubricant will be delivered to the point of lubrication and, in the case of bearings, can substantially shorten the life of the bearing.
The lubricant composition must also exhibit good wettability or spreadability on the surface(s) to which it is applied. One of the problems most frequently encountered with mist lubrication process for large bearings, such as those utilized on rolling and roll neck surfaces, is the lack of uniformity of lubricant distribution over bearing and roll neck surfaces. This lack of adequate lubricant film results in excessive localized wear and premature bearing failure. "Dry neck" or areas of insufficient lubrication on the roll neck are frequently observed disassembly of mist oil lubricated roll bearings. Lubricant compositions that result in all of the bearing and roll neck surfaces being uniformly coated with lubricant significantly prolong bearing life and reduce operating costs. Such compositions are said to possess desirable "plateout" characteristics.
The lubricant compositions of the present invention offer significant advantages over prior art lubricants in terms of cost, physical properties, operating characteristics, and, most importantly, the capacity for utilization in hot rolling steel mill applications where the high temperatures of operation of the mill apparatus would result in the degradation of conventional prior an compositions. These advantages will become apparent based upon the detailed description that follows.